The present invention relates to an amplification apparatus; and, more particularly, to a distortion compensation amplification apparatus capable of reducing power consumption by way of employing an automatic bias control scheme.
In a mobile communications system, a base station transmits an amplified high frequency multi-carrier signal having a plurality of carriers separated from each other by different frequency bands and respectively modulated in a proper way,
Required in a radio amplification apparatus used in such base station of the mobile communications system are low power consumption and a high linearity as well as a high efficiency of a radio transmission circuit incorporated therein. Since an amplification apparatus with a poor linearity produces large distortion, e.g., inter-modulation distortion, which hampers the realization of a normal and a high quality communication, the amplification apparatus for use in the amplification of the multi-carrier signal is required to exhibit good linearity characteristics across the whole frequency band the multi-carrier signal belongs to.
One among various methods for satisfying such a linearity requirement of the amplification apparatus is a feed forward distortion compensation technique. First, referred to as the term xe2x80x98main linexe2x80x99 in the following description is a signal path from an input terminal to an output terminal via a main amplifier, i.e., a signal path for transferring a to-be-amplified signal and an amplified signal.
In the feed forward distortion compensation technique, a distortion detection loop is arranged in such a manner as to couple a signal branched from a position at the back end of the main amplifier to a signal sampled from somewhere in front of the main amplifier on the main line. If the two signals have same amplitudes but with inverse phases, the carrier signals are cancelled out by the above-described signal coupling operation of the distortion detection loop and, then, an error signal corresponding distortion components introduced by the main amplifier and its neighboring circuits can be outputted.
The feed forward distortion compensation technique further employs a distortion compensation loop for re-coupling the error signal outputted from the distortion detection loop to the signal on the main line. The distortion components produced by the main amplifier can be compensated by compensating on the main line a signal delay in the distortion compensating loop and, further by appropriately performing an adjustment on the amplitude and the phase of the signal on the distortion compensation loop or the main line, in such a manner that the distortion components in the signal on the main line and the signal obtained from the distortion compensation loop are controlled to have identical amplitudes but with inverse phases.
The feed forward distortion compensation technique will now be described hereinafter in further detail with reference to FIG. 15.
Referring to FIG. 15, there is provided a conventional distortion compensation amplification apparatus having a feed forward configuration. The feed forward distortion compensation amplification apparatus includes two blocks: one is a distortion detection loop block including a directional coupler 1, a phase shifter 2, an amplitude adjuster 3, a pilot signal generator 4, a coupler 5 and a main amplifier 6; and the other is a distortion compensation loop block including a coupler 8, a detector 9, a phase shifter 10, an amplitude adjuster 11 and an error amplifier 12. The feed forward distortion compensation apparatus further includes dividers 7 and 13, a termination device 14, a coupler 15 and a control unit 18 for controlling each element of the feed forward distortion compensation amplification apparatus.
Within the distortion detection loop block there exist two routes: one is a distortion-free route of axe2x86x92cxe2x86x92exe2x86x92g; and the other is a distortion route through axe2x86x92bxe2x86x92dxe2x86x92f. At the point of xe2x80x9cgxe2x80x9d, only the distortion component is extracted by way of, e.g., inverting the signs of the transfer functions. At this time, the control unit 18 performs an automatic control on the phase shifter 2 and the amplitude adjuster 3 such that a received signal level of the distortion component measured by the detector 9 is minimized.
Another widely employed technique for achieving a good linearity in an amplification apparatus is an adaptive pre-distortion compensation technique (hereinafter referred to as an ADP technique) using a digital signal processing scheme. The ADP technique in general can be classified as a radio signal input type and a base band signal input type. Referring to FIGS. 16A and 16B, there are provided distortion compensation amplification apparatuses of such types.
FIG. 16A shows a distortion compensation amplification apparatus of the radio signal input type. A radio signal is inputted to an input terminal 21 and is successively subjected to a delay circuit 22, a gain control circuit 23 and a phase control circuit 24 and then, amplified at a power amplifier 34. The amplified radio signal is outputted through a coupler 38. Meanwhile, the input radio signal is also converted to a digital data signal by passing through a logarithmic amplifier (LOG AMP) 27 and an A/D converter 28-1, both of which are incorporated in a power meter 29. Afterwards, the digital data signal is provided to a data processing unit 30-1.
Then, the amplified radio data signal outputted from the power amplifier 34 is delivered to a multiplier circuit 33-1 through the directional coupler 38 and combined with the local frequency 32-1 thereat. The coupled signal from the multiplier circuit 33-1 is then subjected to a band-pass filter 31 to have a limited frequency band. Next, the signal with the limited frequency band is inputted to the data processing unit 30-1 through an A/D converter 28-2. The data processing unit 30-1 operates by an algorithm for minimizing a level of the signal passing through the band-pass filter 31, i.e., off-band spectrum components generated by non-linear distortion characteristics of the power amplifier 34.
The data processing unit 30-1 refers to addresses of memory circuits 26-1 and 26-2 based on the digital data signal provided from the power meter 29 to thereby control the gain control circuit 23 and the phase control circuit 24 through D/A converters 25-1 and 25-2, respectively. The contents of the memory circuits 26-1 and 26-2 are determined on a trial and error basis such that the non-linear distortion of the power amplifier 34 becomes minimized.
Referring to FIG. 16B, there is provided a conventional distortion compensation amplification apparatus of the base band signal input type.
Carrier offset circuits 35-1 to 35-4 serve to offset carrier frequencies against respective input digital channel data based on carrier information for each input channel. The carrier offset circuit may be implemented, for example, by a complex multiplier. The outputs from the carrier offset circuits 35-1 to 35-4 are provided to an adder 39, which generates, in response thereto, a combined base band signal to the complex multiplier 36 and a power meter 29.
The complex multiplier 36 controls the phase and the amplitude of the combined base band signal. Thereafter, the thus processed base band signal is subjected to a D/A converter 25-4 and a quadrature amplitude modulation unit (hereinafter, a QAM-MOD) 37, successively, and, then, combined with a signal from a local oscillator 32-2 at a multiplier circuit 33-2. Subsequently, the combined signal is transferred as a radio signal to the power amplifier 34 to be amplified thereat and finally outputted through a directional coupler 38. An algorithm for minimizing non-linear distortion of the power amplifier 34 controls, as in the earlier-described radio signal input type, the complex multiplier 36 based on a power level of the base band signal, to thereby adjust the phase and the amplitude of the base band signal. That is, a data processing unit 30-2 refers to an address of a memory circuit 26-4 based on power level of the base band signal obtained from the power meter 29 to thereby control the complex multiplier 36. The contents of the memory circuit 26-4 which stores complex control values are determined on a trial and error basis such that the non-linear distortion can be minimized.
Recently, the number of mobile subscribers has been rapidly increased. Therefore, in order to cope with such ever-increasing demand a greater number of base stations need to be implemented as well in a same service area. Accordingly, operation costs and maintenance repair costs have been continuously increased and, thus, base station providers have sought to reduce the costs in many ways.
One way to cut the operation costs is to reduce the power consumption.
A transmitter power amplifier of the base station consumes a considerable amount of power at a constant rate regardless of the relative strength of the output signal transmitted by the base station. For instance, in daytime, the RF output power level consumed for a normal traffic load is about 10 watts and DC prime power consumed by the transmitter power amplifier is about 80 to 100 watts. However, in the middle of nighttime when the traffic load becomes light, the RF output power level of the transmitter may be reduced in decrements down to, e.g., about 1 watt due to the power control conducted over the RF output signal as described above. However, since an operation bias point is fixed in the conventional power amplifier, the DC prime power consumed by the transmitter power amplifier in the nighttime is not reduced but still remains unchanged at about 80 to 100 watts as in the daytime.
Accordingly, it would be advantageous to develop a power control technique capable of reducing the DC power consumption of a power amplifier of a base station while maintaining the existing output RF power level. In such a CDMA mobile communications system, it is required to monitor and maintain an RF output signal level as needed and, concurrently, to reduce the DC power level required to generate the target RF output signal level.
It is, therefore, an object of the present invention to provide a distortion compensation amplification apparatus capable of lowering consumption power by way of bias control.
In accordance with a preferred embodiment of the present invention, there is provided a feed forward type distortion compensation amplification apparatus, including: a distortion detection loop having a first phase shifter, a first amplitude adjuster and a main amplifier; a distortion compensation loop having a second phase shifter, a second amplitude adjuster and an error amplifier; means for canceling a first distortion element generated from the distortion detection loop by a second distortion element generated from the distortion compensation loop and generating an output signal; and a bias controller for controlling bias levels of the main amplifier and the error amplifier based on a level of the output signal.
In accordance with another preferred embodiment of the present invention, there is provided an adaptive pre-distortion type distortion compensation amplification apparatus, including: a detection unit for detecting an input power level; a control unit for storing the input power level and a bias level corresponding thereto and generating the bias level in response to the input power level; and a bias control unit for applying a bias to an amplifier unit in response to the bias level.
In accordance with still another preferred embodiment of the present invention, there is provided a distortion compensation amplification apparatus, including: a detection unit for detecting a level of an input signal; an amplification unit for amplifying the input signal; and means for stepwise controlling a bias of the amplification unit based on the level of the input signal detected by the detection unit.
In accordance with still another preferred embodiment of the present invention, there is provided a distortion compensation amplification apparatus, including: an amplification unit for amplifying an input signal; a detection unit for detecting a level of an output signal; and means for controlling a bias of the amplification unit based on the level of the output signal detected by the detection unit.